There's More to Gunpowder Than You Think
January 04, 2019
How do you decide what propellant you’re going to choose in your reloads? The honest answer for most of us is usually two questions: What gives the highest velocity and what gives the best accuracy? That’s a good answer, but there are characteristics of the different propellant types to consider.
So, let us explore the details about propellants that will allow you to make an informed decision the next time you’re wondering which propellant to load for the best possible loads.
How is powder made? Let’s start with how propellants are made. The basic ingredient in most small-arms propellant is nitrocellulose. Nitrocellulose is made by treating cellulose, such as cotton or wood fiber with nitric and sulfuric acid. It is usually stored and processed wet and is quite stable in this condition.
When it’s mixed with solvents and prepared for processing, it looks like bread dough. The nitrocellulose has chemicals added to it to make it chemically stable, increase its energy and control the burn rate. There are a number of shapes of propellant depending on the production method used. Extruded (stick), flake and ball are the most common.
This thick propellant paste is then extruded through a plate with a large number of holes in it. Rotating knife blades on the output side cut the extruded strands into a cylinder of some length. If the rotating knife blades were not used, we’d have spaghetti or stick propellant. Flake propellant is nothing more than a very short cylinder. Ball propellant is cut as a cylinder that is as long as it is wide. These cylinders, carried by water, are then pumped through several thousand feet of pipe. The tumbling action turns the short cylinders into spheres. The so-called “speed,” or how rapidly a propellant produces pressure, is controlled by the size of the grain and the use of chemical burn-rate inhibitors.
Progressive Burn Rates
The term “progressivity” has been thrown around a lot recently in marketing campaigns, but it has not been well defined. As it applies to gun propellants, it’s defined as the burning surface area or amount of gas a propellant can produce compared to the percent of the grain of propellant that has burned.
There are regressive, neutral and progressive burning propellants. The more progressive the power, the higher the performance the propellant will produce. This is what most new propellant products have been about, which include Alliant Power Pro, Hodgdon Hybrid and Hornady Superformance.
Regressive burning means there is constantly less surface area or gas produced as the propellant grain burns. An example of this would be a stick shape with no hole in it or a true spherical propellant.
Neutral burning means the propellant has essentially the same surface area as it burns and produces a constant amount of gas as it burns. This would be a stick propellant with a single hole through it. Virtually all small-arms stick propellant is made with a very small hole through each grain.
Progressive means it’s surface area or gas production rate increases as it burns. Examples would be large cannon propellant with multiple holes or stick and ball propellant that has its burn rate or gas production rate chemically controlled.
You will get the highest performance from the most progressive propellant. It will produce greater amounts of gas for a time as the projectile moves faster down the bore, keeping higher pressure on the projectile and producing higher velocity.
In propellants, there are single- and double-base types. Single-base propellant has its energetic component being only nitrocellulose. To get higher energy content into propellant and therefore more performance, nitroglycerin (NG) is added to the nitrocellulose. Propellant that contains NG is called “double base.”
NG is an explosive and contains lots of energy, but it is not a completely free lunch. Double-base propellants usually do not have the temperature stability that single-base propellants do. They can change performance as the temperature changes.
Double-base propellants will lose pressure and velocity as it gets cold and gain pressure and velocity as it gets hot. Recent advancements in rifle double-base propellants have largely solved this problem. Stick propellants are now being made with the small hole in the middle of the grain covered over by the deterrent coating. This controls the burning of the propellant and improves the temperature performance.
Let’s now look at specific propellant types and what to consider for specific applications.
Virtually all stick propellant used in small arms is single-perforation (hole) grain. It is predominantly used for rifle propellants. A chemical deterrent coating is added to the surface to slow the initial burn to further improve the progressivity. These types of propellants are available in single or double base.
Examples of single-base propellants include the old IMR line, Hodgdon Extreme and Vihtavouri 100 series. These propellants produce good to extremely good temperature performance.
Double-base stick propellants typically have 6 to 8 percent NG. Examples of double-base propellants are nearly the entire Alliant line, the IMR Enduron line, Norma propellants and Vihtavouri’s 500 series. Older double-base propellants typically produce top velocity, but some do not produce very good temperature performance.
Recent developments in both single- and double-base stick propellants have resulted in virtually no sensitivity to temperature. The single-base Hodgdon Extreme propellants are typically the best.
Application of this production technique to the Hodgdon Extreme line, IMR Enduron line and the newer Alliant propellants such as RL 16, 23, 26 and 33 have resulted in single- and double-base propellants that have little to no performance change with temperature.
If you use stick propellants for reloading, one drawback is they typically will not meter as uniformly through a powder measure, unless they are very small in grain size. If you want the best uniformity from stick propellants, you will need to weigh individual charges. If you are loading a lot of ammunition and using a powder measure, this is a consideration.
If you’re a benchrest shooter, competition match shooter or especially a long-range shooter, you should try to use the temperature insensitive propellants. Doing this will give you uniform performance and ballistics regardless of environmental conditions.
Ball, or spherical propellant as it is commonly called, is a bit of a misnomer. Very few ball propellants are actually spheres. With the exception of a couple propellants on the market, most are shaped like a thick pancake.
The propellant starts life as a sphere, but is put through a process called “rolling.” The spheres of propellant are passed through counter-rotating rollers that flatten the spheres. Essentially what is being done is converting a sphere into a flake-type grain to improve the progressivity. Ball propellants span the entire spectrum of applications from shotshell to rifle. Ball propellants are double base because NG is needed for the deterrent process. Rifle-speed ball propellants generally are 10 to 15 percent NG. Pistol and shotshell speeds have 15 to 35 percent NG.
The deterrent coating that is placed on ball propellant is much different than the extruded propellants. The deterrent on ball propellants is actually impregnated into the grain of propellant; it’s not just a coating on the surface.
This results in a propellant grain that steadily burns faster and faster, producing more and more gas as it burns through the outer 50 percent of the grain. The net result of this is that ball propellants can be manipulated by the combination of rolling and the deterrent placement to produce very progressive propellants.
The best examples of this today are the Hornady Superformance propellants. These are highly engineered propellants that produce very high levels of performance. Recently, the ball powder deterrent coating process has been applied to stick propellants to produce very progressive stick propellants. Hodgdon Hybrid 100V is an example of this type of design.
Ball propellants have several advantages over stick propellants. The first is that they flow easily and meter very uniformly through a powder measure. The second is they have higher bulk density than stick types; you can get more charge weight in the case. If you’re having trouble getting enough stick propellant in a case, consider a ball propellant of comparable speed. You will get more powder in the case and achieve the performance you’re after.
Ball propellants have one significant performance drawback. They will usually not provide temperature performance comparable to even the older-design stick propellants. The exception to this is the Hornady Superformance propellants, which were engineered to provide improved temperature performance. Temperature performance is comparable to older-design stick propellants.
If you’re a high-volume loader on a progressive press, interested in top velocity from a given cartridge or are not concerned with velocity variation because of temperature with what you are loading, consider a ball propellant.
In the table, we show some representative changes in performance of older and newer designed single- and double-base stick propellants and ball propellants as a function of temperature.
Flake propellants, for all practical purposes, are nothing more than a very short, stick propellant. Flake propellants can be either single or double base.
Double-base flake propellants can be as high as 40 percent NG. Flake propellants are generally used for pistol or shotshell loads. Some of the flake propellants are manufactured such that they are curved, which causes them to have a lower packing density and lower charge weight in a given case. This makes them ideally suited for some pistol and most shotshell loads.
The very high NG level of some of these propellants improves the cold temperature performance. This is very important to a shotgun shooter while hunting in cold weather. Examples of single-base flake propellants are the Hodgdon Clays line and IMR 800X. Examples of double-base flake propellants are Alliant Bullseye, Reddot, Greendot and Bluedot.
Flake propellants would be a very good choice for many low-pressure pistol or revolver cartridges and many shotshell loads of all gauges.
Care & Storage
With any propellant, extreme care must be taken when caring for and storage of propellants. So, here’s your warning.
Heat is the enemy of propellant. Do not store propellant or ammunition in a high-heat environment for long periods of time. Examples of poor places to store propellant and ammunition would be in your garage during summer.
High heat (above 100 degrees Fahrenheit) will cause two problems: The first is that elevated heat causes the chemical stabilizer in the propellant to deteriorate much faster. After extended periods of exposure to high heat, the propellant can self-ignite.
The second problem with prolonged exposure to high heat is that the deterrent in the propellant can begin to move farther into the propellant grain, especially in ball propellant. This almost always leads to propellant or ammo producing higher pressure than it should. Several days of exposure isn’t a problem, but months of exposure can be problematic.
Hopefully, this gives you a better understanding of propellants and helps you make good choices on which propellant to use for your particular interest.